The present invention relates to a method and apparatus for evaluating a paint filter. More particularly, it relates to a method and apparatus for evaluating the fractional efficiency of a paint filter under real world spray painting conditions.
A well-known manufacturing approach for applying paint onto a surface of an object is spray painting. This technique can be used to coat a wide variety of objects, ranging from large automobile bodies to miniature plastic components, with paint. Depending upon the particular application, a number of different paints can be used, including water-based, solvent-based, acrylic-based, metallic paints, etc., to name but a few.
A typical spray painting operation takes place within an enclosed spray booth. The object to be painted, as well as a paint spray gun, are located within the spray booth. The spray gun receives paint and atomizing air to break up the paint into small particles. These particles, in turn, are directed toward the object to be coated. To clean up the unused paint particles, the spray booth normally includes an exhaust fan positioned behind, or downstream of, the object to be coated, and draws air from inside the spray booth.
During use, paint particles are directed at and adhere to the object surface. It is understood that not all paint particles will attach to the object. Those paint particles that do not deposit on the object are referred to as "overspray". These overspray particles will either deposit on the spray booth walls or continue downstream toward the exhaust fan. To prevent overspray from exiting the spray booth via the exhaust fan, a paint filter is provided upstream of the exhaust fan. Thus, the paint filter forms a vital component of the spray painting process as it prevents undesirable and potentially hazardous release of paint particles into the surrounding environment.
Paint filters are designed to collect airborne paint particles in the form of overspray. Due to diverse paint formulations and applications, a number of different paint filters are available. As is known in the spray painting and paint filtration industries, paint particles are resinous and may include pigments, additives and solvent. For metallic paints, metallic flakes are encapsulated within the particles. Even further, to control emissions of volatile organic compounds, water-based and high-solids paints have been developed. Therefore, depending upon the particular paint application, one particular paint filter may be more or less useful than another.
The performance rating of a paint filter is normally measured by parameters such as pressure drop across the filter, average filtration efficiency, fractional efficiency and loading capacity. To accurately evaluate the filtration performance of a paint filter based upon any of these parameters, a carefully designed testing method must be used. One testing standard, referred to as ASHRAE 52.1, has been prescribed to measure dust spot efficiency, average filtration and loading capacity for air filters. Further, an ASHRAE 52.2 standard has been proposed for evaluating air filters in terms of fractional efficiency. Even more recently, the Environmental Protection Agency has released a proposed testing method, referred to as Method 319, for measuring fractional efficiency of paint filters. Method 319 is similar to ASHRAE 52.2. With each of these testing methods, and others that have been proposed and/or implemented, a paint filter is "tested" by directly exposing the paint filter to particles of a predetermined compound. These compounds include potassium chloride (KCl), sodium chloride (NaCl), oleic acid or dust particles as the challenging particles. While these standard testing approaches have been accepted by the industry, they do not provide a suitable evaluation of paint filter performance for at least two reasons.
First, it is well accepted that paint particle adhesion to a paint filter media varies widely from paint to paint. When the paint particles are drawn through the paint filter, the collection efficiency of the paint filter depends upon paint particle size and adhesion between the particles and the paint filter. Therefore, any test utilizing a challenging compound or particle other than the actual paint used with the paint filter does not provide a true indication of that filter's usefulness for a particular application. For example, a fast drying paint normally uses a fast drying solvent with resulting paint particles being drier and less sticky. A paint filter test utilizing, for example, potassium chloride instead of the fast drying paint will not provide a viable evaluation of the paint filter for the desired paint application. In other words, the standard test will show the paint filter as being more or less efficient than it might otherwise be when used to collect fast drying paint particles.
A second drawback presented by preferred testing methods is a failure to test a paint filter's ability to retain overspray paint particles. As previously described, overspray is paint particles which have deflected off of, or otherwise not adhered to, the object being coated. Through testing, it has been shown that overspray particle diameter is much less than that originally produced by the spray gun. FIGS. 1A-1C provide a graphical representation of paint particle size (solvent based white acrylic enamel) at three different air-to-paint ratios (A/P). As is shown, the paint particle size at the object (or target, FIG. 1A) is much higher than the overspray (or exhaust, FIG. 1C). The standardized testing techniques fail to account for this change in particle size. In other words, standardized testing techniques unrealistically evaluate a paint filter by subjecting the paint filter to spray directly from the spray gun. During use, the paint filter will be required to collect overspray particles, not direct spray. Because the particles associated with the direct spray are larger and more easily retained, a skewed evaluation may result.
Therefore, in view of the above problems associated with accepted filtration testing techniques, a substantial need exists for a method and apparatus for accurately evaluating paint filter efficiency under conditions as close as possible to actual conditions.